M.2 and SATA are storage drive interfaces. The Samsung 970 connects to the M.2 slot. The HDD will connect to the SATA. You can use both as long as you don't connect the HDD to a disabled SATA port (figure which ports are disabled from your motherboard's manual)

It means exactly that. If you install something in the M.2 slot, you can't use two of the SATA ports.
EDIT: If you want a more technical explanation, the chipset has a number of ports peripherals can connect to. It's just that the M.2 slot and two SATA ports use the same chipset ports but they can't share it.

The difference is when the government makes a request to companies. US companies can refuse a request of the government without the government making it harder to do business within it. Nothing significant, or at least public, came against Apple for refusing the FBI's request to remove the "self destruct" feature of the iPhone if too many wrong guesses were made to unlock it.
The allegation is that companies operating in China will comply with government requests, otherwise the government will make sure the company has a hard time doing business in the country.

However it's accessing the same file (so the SSD knows exactly where to look) and it's not touching the file system after the test file was created. Copying files around touches the file system for every file copied.

But again, 4K benchmark tests do not account for file system processing overhead. You can tell robocopy to use more threads to speed up the process: https://pureinfotech.com/robocopy-multithreaded-file-copy-windows-10/

Yes. Besides that, the speed at which the RAM operates at is dictated by the motherboard.
You can, but it's not ideal. See the third post (or second answer) at https://serverfault.com/questions/650426/two-different-intel-xeon-e5-24xx-on-one-dual-socket-motherboard.